Decadal-Scale Changes in Shelf Internal Tides in the Columbia River Plume Area

Columbia River plume waters are rich in silicate and iron, but relatively depleted in nitrate. Mixing of nitrate-rich upwelled water with the plume appears, therefore, to be important to plume primary production. Part of this mixing is accomplished by strong, irregular plume internal tides, and changes in these internal tides could impact productivity. A view of decadal scale changes in the internal tides in the plume area can be obtained through analysis of coastal tidal elevation records, 1925 to date. Coastal tide-gauge signals are difficult to interpret, however, because observed changes may be caused by altered river flow and estuarine morphology, as well as by changes in shelf internal tides. Use of tidal records to diagnose changes in internal tides in the plume area requires, therefore, separation of changes in estuarine tidal dissipation from those in the tidal boundary conditions at the estuary mouth. This can be carried out through analyses of changes in multiple constituents at multiple sites. Observations indicate that there has been about a 0.7 mm/yr increase in M2 amplitude at Astoria, OR since 1925. The diurnal tide has increased at a proportional rate. Since linear diurnal internal tides cannot occur at this latitude, there has clearly been a reduction in bed friction, an idea supported by a simultaneous decrease in the M4/M2 ratio. Changes in estuarine bed friction have been quantified from changes in the spatial distribution of M2 and S2, i.e., from changes in neap-spring variations. Even after removal of the effects of reduced estuarine bed friction, there remains an increase in estuarine M2 amplitude that implies a shift in shelf internal tides. A key issue, whether this shift in internal tides results from a trend in amplitude (perhaps implying a change in vertical mixing) or a phase change (leaving mixing largely unchanged), remains open. The situation in Astoria, where long-term changes in coastal tides are the result of both alteration of the estuary and changes in shelf processes, may be contrasted with that in San Francisco, where M2 has increased since 1900 without any increase in K1; the latter changes may be dominantly due to changes in shelf processes.